The present invention relates to a coating method.
A conventional ink-jet printer selectively discharges small droplets of a liquid ink in accordance with recording signals or controls discharge of such ink droplets so as to form an image.
In a conventional hot-melt transfer printer, a sheet with a hot-melt ink coated thereon and printing paper are superimposed on each other. A recording head comprising a thermal element or a stylus electrode is urged against the back surface of the ink sheet. A recording signal voltage is applied to the recording head to transfer hot-melt ink onto the printing paper.
In a conventional heat-sublimation printer, an ink sheet of a heat-sublimation dye and an image carrier sheet of polyester or the like are superimposed on each other. A thermal head is urged against the back surface of the ink sheet. The heat-sublimation dye is sublimated in accordance with the recording signal and transferred to the image carrier sheet.
Since these printers are of non-impact type, operation noise is small, high-speed printing can be performed, the mechanism is simple, and a color printer can be easily provided. However, images formed by such printers are not free from disadvantages. For example, an image printed by an ink-jet printer cannot have high print density and tends to have relatively poor color clarity. The image also has poor water resistance.
FIGS. 1A and 1B are views for explaining such disadvantages in an ink-jet printer. Referring to FIG. 1A, reference numeral 1 denotes printing paper; 2, a printed image segment; and 3, irregularly reflected light. The paper 1 shown in these figures is slightly uneven. Since the ink slightly covers such paper, the ink layer is also coated in an uneven pattern. Therefore, light is irregularly reflected by the ink layer. Irregularly reflected light lowers the image density and degrades color clarity.
FIG. 1B shows a state wherein a printed image has been partially wetted with water. Referring to FIG. 1B, reference numeral 2 denotes a printed image segment; and 4, a water droplet. When the water droplet 4 becomes attached to the image, the dye dissolves and causes damage to the image.
An image formed by a hot-melt transfer printer is subject to the problem of degradation in image quality due to deformation of image dots by an external force such as rubbing. FIG. 1C is an enlarged sectional view of such an image. Referring to FIG. 1C, reference numeral 5 denotes printing paper; 6, a hot-melt ink image; 7, a printed image segment subjected to an external force; and 8, a printed image segment enlarged due to deformation by the external force. FIG. 1D is a plan view of the image shown in FIG. 1C. In FIG. 1D, reference numeral 9 denotes an arrow that indicates the direction along which the image surface has been rubbed. The rubbed image segment 8 is blurred from a pixel end and the image area is increased. Thus, the rubbed segment 8 has a higher print density than the remaining image segment and is therefore enhanced.
Back transfer occurs when a sheet of paper is placed on an image formed by the hot-melt thermal printer and one writes on this sheet of paper. The hot-melt ink is transferred to the paper's back surface.
Ink dye used in a heat-sublimation printer is basically unstable in the presence of heat and may cause discoloration of an image over time. In order to prevent this, dye having a high sublimation temperature must be used. However, in this case, excessive thermal stress is put on a recording head, and the life of the head is shortened. In addition, since the current capacity of the head drive circuit is increased, the overall drive circuit, including a power source, is rendered large in size. When the sublimation temperature of the dye is decreased in view of such problems, the stability of printed images is significantly degraded.
As described above, printers of various types for forming images by selectively applying a colorant on printing paper can provide only unsatisfactory image stability and density. The stated disadvantages have prevented these printers from being commercially worked.
Some countermeasures have been proposed in view of these problems. For example, Japanese Patent Disclosure No. 59-39575 discloses an apparatus which has a laminater for laminate coating the image-carrying surface of a printing paper sheet with a transparent film sheet. However, a material which can form a film by lamination is limited to a film-forming material. Therefore, the surface of printing paper laminated with such a film has a surface state very different from a normal paper surface. In addition, no further writing can be done on the laminated surface. The printing paper must be sufficiently dried before lamination, the lamination apparatus is hard to design, and the field for application of such an apparatus is limited.
Japanese Patent Publication No. 56-2022 discloses a printer wherein an ink droplet path from an ink-jet nozzle to printing paper is filled with the mist of a glossy liquid. The gloss of the paper is thereby improved without causing clogging of the ink-jet nozzle. However, it is difficult to form a stable mist in air without dispersion and to maintain it at a level capable of improving gloss.
In a technique disclosed in Japanese Patent Publication No. 56-26312, an image is formed by a heat ray-absorbing ink. The image is irradiated with heat rays to heat only the image portion. Image ink heated to a high temperature in this way is brought into contact with a coating agent so as to selectively coat the image portion. However, in this method, heat is easily diffused from the image ink and the heated image portion is easily cooled. Since the heat capacity of the image ink is very low, it is almost impossible to melt the coating agent and it is assessed to be very difficult to put a stable coating operation into practice.